For the purpose of connecting a number of customers to a core network to enable data transmission, it is a well known approach to rely on an optical access network in the form of a passive optical network (PON).
In such an optical access network, a customer connects his equipment to an optical network unit, which may be also called an optical network terminal, wherein the optical network units of the access network are connected via optical fibres to a remote node. The remote node in turn is connected via an optical feeder fibre to an optical line terminal (OLT) located in the so called central office. The optical line terminal in turn forms the interconnection to the core network.
In a WDM optical access network, the different optical network units transmit different upstream signals at different respective wavelengths to the remote node, which combines these upstream signals and transmits them via the feeder fibre to the optical line terminal. In The WDM optical access network, each optical network unit transmits its respective upstream signal at a separate, individual wavelength.
The concept of WDM may be extended to the concept of TWDM, wherein the concept of WDM is established in conjunction with the concept of time division multiplex (TDM). In such a TWDM optical access network, two or more optical network units of a same group share a same wavelength for their respective upstream signals, wherein the data transmission in the upstream direction on this same wavelength is split among the group of optical network units via TDM.
Data transmission in the downstream direction from the OLT towards the optical network units takes place, such that an overall downstream signal comprises a number of different downstream wavelengths, preferably four or eight different downstream wavelengths. Each of the downstream wavelengths is preferably split into frames via TDM splitting. Thus, one downstream signal of one downstream wavelength is assigned to a number of optical network units.
At the feeder fibres end, the remote node splits the downstream signal of the downstream wavelengths onto the different fibres leading to the different optical network units by means of power splitting.
At the optical network units, data reception is carried out, in that each optical network unit uses a tunable filter for receiving that downstream signal of that downstream wavelength assigned to the respective optical network unit. Furthermore, in the case that a downstream signal of a specific downstream wavelength is split into frames via TDM splitting, the optical network units selects those data frames that are assigned to the respective network unit.
The optical network units transmit their respective upstream signals in the upstream direction using a form of amplitude modulation called on/off keying. Such on/off keying leads to a broadening of the transmission wavelength, wherein this broadening is not negligible.
For reception of the upstream signals at the OLT, the different upstream signals at the different respective wavelengths are filtered by respective filters with fixed transfer functions. The filters may be provided as a combined optical filter. The respective filtered signals are then provided to respective receivers for deriving from the respective upstream signals respective data streams, which then in turn may be used for further data transmission towards the core network.
Since the different upstream signals with their respective different upstream wavelengths are filtered at the OLT using fixed filter functions in the optical domain, it is a critical aspect, that the optical network units actually transmit their upstream wavelengths at such wavelengths that are assigned to them and at which the respective upstream signals are able to pass through the corresponding optical filters' transfer functions in order to reach a respective designated receiver within the OLT.
An approach to ensure that an optical network unit transmits its upstream signal at the respective assigned proper wavelength is to foresee a tuneable transmitter within the optical network unit, wherein the transmitting wavelength may be monitored by control devices placed within the optical network unit. Such a solution may be cumbersome and expensive.
The document EP 2675089 A1 discloses an optical line terminal, in which an upstream signal is provided to a receiving unit as well as tapped for providing a part of the upstream signal to a control unit, which determines a wavelength of the upstream signal for an iterative wavelength tuning process in which a wavelength of an optical network unit is tuned.
The document US 2004/188600 A1 discloses a periodic filter with filtering ranges having respective peak wavelength for monitoring a wavelength of a tuneable laser system.
The document U.S. Pat. No. 5,729,347 A discloses optical filters that have monotonically increasing filter values for an increased wavelength.